A few things when switching to Ubuntu weren’t working out of the box as I’d liked. Some still aren’t. The maybe not that trivial or obvious fixes are listed here.

Evolution Spamassassin Plugin

The bogofilter plugin must be disabled, else spamassassin plugin won’t work. Only one of the two spamfilter plugins may be enabled.
I’ve installed spamassassin and spamc packages with service spamd running.

GnuCash HBCI Support

The Ubuntu and Debian packages don’t include HBCI/FinTS support used for online banking. I had already installed aqbanking and QBankManager, but meanwhile there’s the apt repo geole.info which provides the GnuCash HBCI functionality for Debian Etch and Ubuntu Feisty.
This should install all required packages for GnuCash HBCI from geole:

aptitude install gnucash-aqbanking-hbci

Amarok German Localization

After I installed language-pack-kde-de Amarok speaks German.

Hide Gnome Printer Applet

The tray icons for installed printers are permanently shown and eat up my precious screen space. I think the following shouldThis will get the behaviour I know from Fedora, that the icons are only displayed when I ran a print job. But I haven’t tested that yet!
System > Settings > Sessions: untick “print queue applet”

Azureus with Sun Java

I’ve installed Sun JDK (also containing the JRE) and set /usr/bin/java to point to the Sun version. Azureus for Sun Java installed via apt-get was crashing regularly. Using the binary release from the Azureus project website did the trick.
I even rebuilt the Ubuntu Azureus package, but the result was as unstable as with the apt version.